If I leave my computer locked for a long time, when I give my password to unlock, the plasma splashscreen (black screen with the spinner etc..) appears (but I'm already connected!) and I have to do CTRL+Alt+F2 (go to tty2) then CTRL+Alt+F1 (to come back to tty2, where my X Server is started) to make the splashscreen go. Then I cannot start X applications: $ kmail No protocol specified QXcbConnection: Could not connect to display :0 zsh: abort (core dumped) kmail At the end, I need to reboot to use my computer.
Your Xauth cookie has been deleted. To test if it's that run:echo $XAUTHORITY ls that file reproduce that bug ls that file again. There was another guy on bugzilla a few months ago with the same thing. Unless you have evidence to the contrary. It isn't being deleted by Plasma.
SuSE? See linked report it identifies the problem. *** This bug has been marked as a duplicate of bug 370608 ***